Several years ago, there was introduced to the explosive blasting field a new detonation system for detonating blasting charges, particularly a series or pattern of explosive charges disposed underground in bore holes drilled in the overburden above coal or some other mineral to be mined using conventional surface mining techniques, in which instead of detonating the explosive charges by means of electrical impulses transmitted from an electrical source through a network of wire conductors to electrically detonatable fuses associated with the explosive charges, detonation is accomplished by means of an explosive gas mixture. In this gas-initiated blasting technique, an oxidizing gas, such as oxygen, and a fuel gas, such as methane or the like, are blended in proportions necessary to form a combustible mixture and the mixture is delivered by means of small plastic tubing having, for example, an internal diameter of about 1/8 in., extending to the individual explosive charges and connected to special fuses forming part of the system and adapted to be detonated by combustion of the gas blend propagated through the tubing. After the tubing network is filled with the gas mixture, the latter is ignited within a combustion or ignition chamber communicating with the beginning of the tubing network, which chamber is equipped with a spark generating source, such as a battery operated spark plug or more preferably, a piezoelectric crystal igniter. Such a crystal igniter, as is known in the art, is capable of producing in response to mechanical impact alone an electrical spark of considerable intensity and easily sufficient to initiate combustion of the mixture without the need for a battery or like electrical source. Thus, when the gas mixture in the igniter chamber is ignited, a detonation wave is propagated throughout the tubing network to the fuses to detonate the same and thereby detonate the explosive charges themselves, either directly or by way of booster and primer charges as is known in that field. This system is disclosed in the following U.S. Pat. Nos. 3,885,499 granted May 27, 1975; 3,939,772 granted Feb. 24, 1976; 4,056,059 granted Nov. 1, 1977 and 4,073,235 granted Feb. 14, 1978, all assigned to Hercules, Inc. of Wilmington, Del., which markets components used therein under the trademark "Hercudet".
The detonation gas-activated blasting system briefly summarized above necessarily requires the utilization of thousands of feet of small flexible tubing, preferably of inexpensive plastic, such as polyethylene, including a main trunk line extending from a gas delivery unit and then branch lines as necessary to connect with the fuses for the explosive charges which are distributed usually in a pattern over the area from which the overburden is to be explosively dislodged. The number of explosive charges in the blasting pattern will, of course, vary according to the particular circumstances but can easily range from several dozen to about a hundred, all of which are to be detonated in a single blasting operation. However, the activation of the charges themselves usually does not occur precisely simultaneously but is caused to occur according to a carefully worked out delay sequence in intervals in the range of about 10-1000 milliseconds so as to reduce the force of the overall explosion to within tolerable limits, the delay being accomplished either through varying length of tubing braid lines or through time-delay devices built into the individual fuses in the known way.
The detonation gas delivery unit obviously must be portable for transportation to the blasting area and have sufficient gas volume capacity to fill the entire tubing network including trunk, branch and other line sections. The total length of the tubing network can easily reach several tens of thousands of feet, and it has been demonstrated in practice that detonation wave propagation from the common ignition point throughout the network is effective for aggregate tubing lengths well in excess of forty thousand feet. At lengths of this order, the total volume of the network is approximately 100 liters of the detonation gas blend which fixes the minimum capacity of the delivery unit for a single blasting operation.
For the protection of the public, detailed regulations are observed for the blasting industry which, among other things, requires that after an initial blasting warning signal has been given, for example, by means of a horn or siren throughout the actual blasting area and vicinity, the blasting step must be in fact completed within a reasonable time. It has been generally accepted that fifteen minutes or so meets this standard. Since the detonation gas mixture is itself combustive, introduction of the mixture into the tubing network prior to the warning signal would be unsafe; consequently, the delivery unit must accomplish the task of filing the tubing network within a relatively brief period of time consistent with completion of the blasting operation within an acceptable period after issuance of the required warning. For example, for a network with a total volume of about 100 liters, the delivery unit should have a feed rate capability of at least about 10 liters/minute, allowing the network to be filled in about ten minutes or so.
To this end, the oxidizing and fuel gases can and are supplied under considerable pressure, but this pressure is limited by the construction of the tubing arrangement itself. This arrangement includes in addition to the hollow plastic tubing various types of tubular fittings or couplings, such as L's, T's, etc., adapted to receive by a pressure fit the ends of lengths of tubing and thus permit the tubing network to be assembled to connect with the specific arrangement of explosive charges over the particular terrain in which the explosion is to take place. The tubing network must for practical reasons be adapted for assembly on site with a minimum of effort and without special time-consuming appliances or other measures; therefore, the connection and disconnection of the tubing ends with the fittings must be possible by hand. This requirement restricts the permissible tightness of the pressure fit between tubing and fittings, and hence the maximum allowable gas pressure in the network, since that pressure cannot be so great as to create a risk of blowing apart the connections of the tubing ends with the fittings.
The delivery of the detonation gas to the tubing network for the blasting charge pattern must be carried out after the charges have been placed within their bore holes and are hence inaccessible even for inspection, much less for re-connection of the tubing. If any charge should become separated from its line and detonation nevertheless effected, the result would be the presence of a so-called "loose charge", i.e., an explosive charge which failed to undergo detonation and remains in a potentially dangerous condition below ground. The regulations of the blasting industry require that all "loose charges" be recovered so as to avoid subsequent danger to workers in the area. Such recovery is expensive and time consuming inasmuch as the usual practice in placing the explosive charge is to cover the charge with earth after the same has been dropped into the bore hole and a "loose charge" must literally be dug out of the ground to be recovered. With the presently available "Hercudet" blasting initiation system, the maximum delivery pressure is approximately 40 psig.
In addition to the maximum allowable delivery pressure for the detonation gas, the proportions of the oxidizing and fuel gases employed therein have to be regulated within quite close limits to insure the creation of a mixture of oxidizing and fuel gases susceptible to combustion by spark initiation. While the specific mixture will depend on the particular gases employed, for the most commonly used gases, oxygen and a 50--50 mixture of methane and hydrogen by volume, the blend should contain at least about 50% by volume oxygen and preferably 60% but not more than about 70-75% to exhibit satisfactory combustion properties.
There are commercially available pressure regulated flow valves which are effective to supply a gas flow at a given outlet pressure and include means for sensing the outlet pressure therefrom and for adjusting the valve opening in accordance with the thus sensed outlet gas pressure. However, the effective operation of such pressure regulated valves, and indeed many other types of gas metering devices, depends upon the maintenance of a certain minimum pressure drop or differential across the valve, i.e., a certain minimum difference between its inlet and outlet pressures. It will be apparent that if such regulator valves are employed to form and feed a gas blend into a network of fine tubing at least several thousand feet in length, as described above, the tubing network will inevitably offer a substantial resistance to the flow of the pressurized gas therethrough, due to boundary layer effects, which resistance will appear as a significant back pressure acting against the outlet of the valve. Presently available pressure regulated valves are incapable of satisfactory operation against a back pressure approaching the delivery pressure, that is, when the pressure drop across the valve drops below its design limit, which is typically about 5 psi. Under the latter condition, the operation of the valve becomes unstable and thus unreliable in terms of precise metering action and control is, therefore, lost over the make-up of the detonation gas mixture. Moreover, it is virtually impossible in a practical sense to adjust a pair of such valves operating in parallel to blend two or more gases at exactly the right set points to maintain the proper gas blend, for example, 40% fuel gas and 60% oxygen, and the nature of these valves is such that when they are operated in this way, the valve having a setting exceeding its exact correct point relative to the setting of the other valve becomes pre-eminent in the operation of the valve array. As a result, the supply of the gas delivered by that valve gradually increases, while the supply of the gas passing through the other valve gradually decreases until eventually only one gas is being supplied.
Finally, the delivery unit must satisfy several practical requirements among the most important of which is the ability to operate reliably at relatively low temperatures, i.e., well below freezing. The economics of the blasting industry are such that blasting cannot be suspended on account of cold weather and hence the delivery unit must be capable of reliable operation during cold weather as well as warm weather. Also, the unit must be relatively simple to operate since although blasting technicians may be fully competent in their field, they are not trained in the handling of complex instrumentation and complexity also tends to introduce not only a greater risk of unreliability but of improper operation as well, neither of which are tolerable for blasting purposes.